The present invention relates to a metal-air power supply and more particularly relates to a metal-air cell or a metal-air battery with a load responsive air door.
Generally described, a metal-air cell includes one or more oxygen electrodes separated from a metallic anode by an aqueous electrolyte. A metal-air cell also may include one or more oxygen electrodes that cooperate with metallic anode particles suspended in a paste-like electrolyte. During operation of the metal-air cell, such as a zinc-air cell, oxygen from the ambient air and water from the electrolyte are converted to hydroxide ions at the oxygen electrode. Zinc is oxidized at the anode and reacts with the hydroxide ions. This electrochemical reaction releases water and electrons so as to provide electrical energy.
Metal-air cells have been recognized as a desirable means for powering many types of portable electronic equipment, such as personal computers, camcorders, telephones, and the like. As compared to conventional electrochemical power sources, metal-air cells provide relatively high power output and long lifetime with relatively low weight. These advantages are due in part to the fact that metal-air cells utilize oxygen from the ambient air as the reactant in the electrochemical process as opposed to a heavier material such as a metal or a metallic composition.
One concern in designing metal-air cells is to provide a sufficient amount of oxygen to operate the cells at their desired capacity while also preventing too much oxygen from reaching the cells during periods of non-use. Isolating the cell during periods of non-use minimizes the detrimental impact of humidity, especially while the air moving device is not operational. A metal-air cell that is exposed to ambient air having a high humidity level may absorb too much water through its oxygen electrode and fail due to a condition referred to as xe2x80x9cflooding.xe2x80x9d Alternatively, a metal-air cell that is exposed to ambient air having a low humidity level may release too much water vapor from its electrolyte through the oxygen electrode and fail due to a condition referred to as xe2x80x9cdrying out.xe2x80x9d
The transfer of air and water into and out of a metal-air cell can be described in terms of an xe2x80x9cisolation ratio.xe2x80x9d The xe2x80x9cisolation ratioxe2x80x9d is the rate of the water loss or gain by the cell while its oxygen electrodes are fully exposed to the ambient air as compared to the rate of water loss or gain by the cell while its oxygen electrodes are isolated from the ambient air except through one or more limited openings. For example, given identical metal-air cells having electrolyte solutions of approximately thirty-five percent (35%) KOH in water, an internal relative humidity level of approximately fifty percent (50%), ambient air having a relative humidity level of approximately ten percent (10%), and no fan-forced circulation, the water loss from a cell having an oxygen electrode fully exposed to the ambient air is compared to a similar cell positioned within a housing with limited air access. An isolation ratio of over a hundred (100) to one (1) may be expected depending upon the design of the housing.
Isolating the cells during periods of non-use also minimizes the self-discharge and leakage or drain current. Self-discharge can be characterized as a chemical reaction within a metal-air cell that does not provide a usable electric current. Self-discharge diminishes the capacity of the metal-air cell to provide a usable electric current. Self-discharge occurs, for example, when a metal-air cell dries out and the zinc anode of oxidized by the oxygen that seeps into the cell during periods of non-use. Leakage current, which is synonymous with drain current, can be characterized as the electric current that can be supplied to a closed circuit by a metal-air cell when air is not provided to the cell by an air moving device.
One drawback with the current design of metal-air cells is that the cells tend to be somewhat larger in size than conventional electrochemical power sources. This size constraint is caused, in part, by the requirements of having a metallic electrode, an air electrode, an electrolyte, a cell casing of some sort, and an air manager or an air passageway of some sort to provide the reactant air to the cell. These elements all take up a certain amount of valuable space that could be used for the battery chemistry.
For example, a multiple cell metal-air battery pack housing traditionally has at least one air inlet passageway and at least one air outlet passageway positioned adjacent to an interior fan. The air passageways are generally sealed with mechanical air doors to prevent the transfer of air and humidity into or out of the housing during periods of non-use. An example of a mechanical air door system is shown in U.S. Pat. No. 4,913,983 to Chieky. This reference describes the use of a fan to supply a flow of ambient air to a pack of metal-air cells within the battery housing. When the battery pack is turned on, the mechanical air doors adjacent to an air inlet and an air outlet are opened and the fan is activated to create the flow of air into, through, and out of the housing. The air doors are then closed when the battery is turned off to isolate the cells from the environment. Although the mechanical air doors may limit the transfer of oxygen, water vapor, and contaminates into and out of the housing, such mechanical air doors add complexity to the battery housing itself and, inevitably, increase the size and cost of the overall battery pack.
Further, the air moving devices, such as the fan used in Chieky, are generally bulky and expensive relative to the volume and cost of the metal-air cells. Although a key advantage of metal-air cells is the high energy density resulting from the low weight of the oxygen electrode, this advantage is compromised by the space, weight, and power required by an effective air-moving device. Space that otherwise could be used for battery chemistry to prolong the life of the battery must be used to accommodate an air-moving device. Likewise, the fan also draws a certain amount of power to operate. This loss of space and power can be critical to attempts to provide a practical metal-air cell in a small enclosure such as the typical xe2x80x9cAAxe2x80x9d cylindrical size now used as the standard in many electronic devices.
There is a need, therefore, for a metal-air cell and/or battery pack that is as small and compact as possible, that maximizes the volume available for battery chemistry, and that provides adequate power with an adequate isolation ratio. These advantages must be accomplished in a metal-air cell or battery pack that provides the traditional power and lifetime capabilities of a metal-air cell in a low cost, efficient manner.
The present invention is directed towards a passive, load responsive air valve for a metal-air cell or battery. Advantageously, the present invention thus provides air access to the air electrode of a metal-air cell based upon the operating conditions of the cell. By creating air access in response to the load conditions on the cell, such as the internal current or the internal pressure, the present invention provide reactant air to the cell without the need for an air mover or an air manager with its own control system. Rather, the present invention uses devices such as shape memory alloy elements, bi-metal elements, diaphragms, and the like to provide mechanical action without the use of electronic control systems.
One embodiment of the present invention includes the use of a metal-air cell having a cell casing, an air electrode positioned within the cell casing, and means for providing air to the air electrode when a predetermined load is placed on the cell and for substantially isolating the air electrode when the load is not placed on the cell. The metal-air cell further includes a negative terminal in communication with a metallic zinc anode and a positive terminal in communication with the air electrode. The air providing means is positioned adjacent to the cell casing and in proximity to the air electrode.
The cell casing includes a disk with one or more air apertures positioned adjacent to the air electrode. An air door is positioned on the disk adjacent to the air apertures. The air door is sized to cover substantially the air apertures. The air door may include one or more air shutters rotating about a central hub. A shape memory alloy wire is connected to the disk and to one of the air shutters. The shape memory alloy wire completes a circuit between the positive terminal and the negative terminal. When a load is applied to the cell, the circuit warms the shape memory alloy wire. The shape memory alloy wire therefore rotates the air shutters to expose the air electrode to the ambient air. A spring is also connected to the disk and to the air shutter. When the load is removed, the spring rotates the air shutter back so as to cover substantially the air aperture.
In another embodiment, the air door includes a shape memory-alloy plate. The plate completes a circuit between the positive terminal and the negative terminal. When a load is applied to the cell, the circuit warms the plate. The plate therefore changes shape so as to expose the air electrode to the ambient air. A spring also may be connected to the cell casing and to the plate. The spring forces the plate back so as to cover substantially the air aperture when the load is removed from the cell. A plurality of shape memory alloy plates and a plurality of springs may be used.
In another embodiment, the air door includes a pair of air shutters. Each of the air shutters is connected to the disk by a hinge. A shape memory alloy wire is connected to each of the pair of air shutters. When a load is applied to the cell, the circuit warms the wire such that the wire opens the air shutters to expose the air cathode to the ambient air. A pair of springs is connected to the air shutters. The springs force the air shutters back so as to cover substantially the air aperture when the load is removed from the cell.
In another embodiment, the air door includes a bi-metal element. The bi-metal element is placed in a circuit between the positive terminal and the negative terminal. The bi-metal element moves to expose the air electrode to the ambient air when a load is applied to the metal-air cell. The bi-metal element may be a bi-metal strip or a bi-metal spiral.
In a further embodiment, the metal-air cell includes an air plenum positioned adjacent to the air electrode. The means for providing air to the air electrode when a predetermined load is placed on the metal-air cell and for substantially isolating the air electrode when the load is not placed on the metal-air cell include a diaphragm. The diaphragm may include an air aperture therein. As a load is applied to the metal-air cell, a partial vacuum builds within the air plenum. The partial vacuum causes the diaphragm to expand and air to pass through the air aperture until the partial vacuum dissipates. A plurality of air apertures or isolating air apertures may be used. A spring also may be attached to the diaphragm. The spring may force the diaphragm to contract after the partial vacuum has dissipated.
In a further embodiment, the diaphragm includes a central aperture substantially covered by a disk. A spring is attached to the disk. As a load is applied to the metal-air cell, a partial vacuum builds within the air plenum and causes the disk to pull away from the diaphragm. Air passes through the central aperture until the partial vacuum dissipates. The spring then forces the disk back to cover substantially the central aperture.
In another embodiment, the metal-air cell includes an air electrode, a metallic anode, a negative terminal, a positive terminal, a cell casing, and a passive air manager. The passive air manager includes an air door positioned adjacent to the cell casing in proximity with the air electrode. The air door is positioned in a circuit between the negative terminal and the positive terminal of the cell. The air door changes its shape when the current running through the circuit reaches a predetermined amount, thereby exposing the air electrode to the ambient air. A plurality of air doors may be used. The air doors may use a shape memory alloy element, a bi-metal element, a diaphragm, or similar devices.
In another embodiment, the passive air manager includes one or more air doors positioned adjacent to the cell casing in proximity with the air electrode. An air door actuator is attached to the air doors. The air door actuator is positioned in a circuit between the negative terminal and the positive terminal of the cell such that the air door actuator opens the air doors when the current running through the circuit reaches a predetermined amount. The air door actuator may be a shape memory alloy element or a similar device.
In another embodiment, the passive air manager includes a diaphragm positioned adjacent to the cell casing in proximity to the air electrode. The diaphragm includes an air aperture positioned therein. When a load is applied to the metal-air cell, a partial vacuum builds within the air plenum that causes the diaphragm to expand. Air then pass through the air aperture until the partial vacuum dissipates. A plurality of air apertures or isolating air apertures may be used. A spring may be attached to the diaphragm. The spring forces the diaphragm to contract after the partial vacuum has dissipated.
In another embodiment, the air aperture includes a central aperture substantially covered by a disk. A spring is attached to the disk. As a load is applied to the metal-air cell, a partial vacuum builds within the air plenum. The partial vacuum causes the disk to pull away from the diaphragm and air to pass through the central aperture until the partial vacuum dissipates. The spring then forces the disk back to cover substantially the central aperture.
Other objects, features, and advantages of the present invention will become apparent upon review of the following detailed description of the preferred embodiments of the invention, when taken in conjunction with the drawings and the appended claims.